The South Boston St. Patrick’s Day parade wound its colorful way through the city today past thousands of cheering, chanting, high-fiving spectators, out for a good time — and a celebration of Irish heritage — on a bright, chilly day.

The crowd on Broadway near the start of the parade was a sea of green — sporting everything from green feather boas and leis around their necks to green top hats and antennae with clovers at their tips.

The marchers included everyone from a Coast Guard contingent to the Ghostbusters who followed directly behind them to the Lexington Minute Men reenactors, who startled the crowd periodically by letting off a thunderous volley from their muskets.

The marchers also included a group of Boston Marathon bombing survivors, who were trailed by an MIT police cruiser, dispatched in memory of Officer Sean Collier, who was allegedly slain by the Marathon bombers several days after the terror blasts.

The crowd chanted “Boston Strong,” the slogan coined for Boston’s post-attack resilience, as the marchers went past.

At the end of the parade route, near Andrew Station, college students whooped and batted green beach balls over they head as they jostled for good viewing positions along the street.

As the parade wound down, revelers high-fived police officers and shouted along to the Dropick Murphys’ “Johnny I Hardly Knew Ya,” which blared from speakers on the sheet metal workers’ union float. A person dressed as Mickey Mouse in green suspenders waved from a second-floor balcony on Dorchester Street.

The festivities began after an impasse was reached on a bid by gay rights activists to have gays march openly in the parade. Mayor Martin J. Walsh. son of Irish immigrants, was conspicuously absent, after saying he would not participate because of gays’ exclusion.

Maryann Noonan, 28, Gern Noonan, 53, Mairead O’Flynn, 67, and Breda Downes, 63, had come all the way from Ireland to experience the holiday in Boston.

While sipping on Starbucks coffee drinks (decorated with the obligatory green whipped cream), they discussed how impressed they were with how festive and friendly Bostonians were.

“It’s such a beautiful city,” Gern Noonan said.”Everyone wears so much green.”

The woman agreed that although Monday’s parade in Dublin is professionally organized, it is not as long as the one in Boston, nor is there as much celebrating.

Diogo Azevedo, 22, was looking forward to his first parade. He and four other friends came to Boston from Rider University in New Jersey. The group of friends counted down to their trip from February, using SnapChat messages and paper shamrocks hung on their walls.

“I’m most excited to see all of the different floats,” he said.

In the Cornerstone Pub, near the start of the parade, the owner decided not to serve any Sam Adams beer, because the Boston Beer Co., which makes the brew, had said it would not participate in the parade due to gays’ exclusion.

Ross Cloutier, 32, of Bellingham, said he and his wife were not going to let politics disrupt their day, but he applauded the owner’s decision not to serve the beer. Cloutier is usually a Sam Adams drinker, he said, but not today.

“Good for them. If you believe in something strongly, you stand by it,” Cloutier said.

The parade, which has been held since 1901, winds through 3.7 miles of the neighborhood, a long-time Irish-American enclave that is seeing an influx of newcomers.

After weeks of discussions with the organizers of the parade, Walsh announced this morning that he would not take part in the event.

“I’m disappointed that this year, I will be unable to participate in the parade,” he said in a statement. “As mayor of the city of Boston, I have to do my best to ensure that all Bostonians are free to participate fully in the civic life of our city. Unfortunately, this year, the parties were not able to come to an understanding that would have made that possible.”

For two decades, gay men and lesbians have been excluded from openly marching in the parade. Walsh worked until the last minute to bring the statewide gay rights organization MassEquality and parade organizers from the Allied War Veterans Council to an agreement that would have changed that but to no avail.

A second, alternative parade organized by Veterans for Peace follows a mile behind the traditional parade and has, in the past, included many LGBT groups.

Carlos Arredondo, the good Samaritan who was the subject of one of the iconic pictures of Marathon bombings, was named earlier this month a grand marshal of the alternative parade and immedately called for both the traditional and alternative parades to be merged.

Police said last week they would have a stepped-up presence at the parade because of the Marathon bombings last April 15. But they also emphasized that they have a zero tolerance policy on alcohol and urged people to “celebrate responsibly.” Past parades have been marred by arrests for disorderly conduct and citations for drinking in public.

The police radio crackled late this afternoon with reports of drunken persons and fights. But no arrests had yet been officially logged, a spokeswoman said.